1. Field of the Invention
The present invention relates to an image forming apparatus that forms an image by charging an image bearing member by a charging device having a bias voltage applied thereto and supplying a developer to the image bearing member by a developer carrying member having a bias voltage applied thereto.
2. Description of the Related Art
A conventionally known image forming apparatus irradiates a uniformly charged photosensitive member with laser light to form an electrostatic latent image thereon, develops the electrostatic latent image with toner carried by a developing roller to form a toner image, and transfers and fixes the toner image to a transfer material to form an image thereon.
In the conventional image forming apparatus, a developing device which forms a toner image on the photosensitive member includes, for example, an agitating member which agitates toner while transporting the toner, a layer regulating member which regulates a toner layer on the developing roller, and a toner remaining amount detection element, in addition to the developing roller. The toner in the developing device reaches a predetermined charge amount due to the friction charging caused by the agitation by the agitating member and the rotation of the developing roller. Then, after a toner layer is formed uniformly by the layer regulating member, the toner is transported to a developing region opposed to the photosensitive member by the rotating developing-roller to develop an electrostatic latent image with or without contact to the photosensitive member.
The development of toner depends upon a toner charge and a developing electric field. If the toner charge is high, the toner can be developed onto the photosensitive member even when the developing electric field is low.
Further, the characteristics such as chargeability of toner that is a powder particle are varied depending upon respective toner particle diameters. Therefore, all the toner particles are not charged by friction to a desired charge amount, and the charge amount has a distribution (a certain range) with a desired charge amount as a peak.
Further, when the toner in the developing device is supplied to the photosensitive member and the toner is consumed for image formation, toner is replenished from a developer replenishment container to the developing device according to the detection result of the toner remaining amount detection element. For example, as described in Japanese Patent Application Laid-Open No. H05-46027, at a time of setting a developing device to be used for the first time, an operation of replenishing toner to the developing device containing no toner is performed. Specifically, at a time of a toner install (setting sequence) for replenishing toner to the developing device containing no toner, toner continues to be replenished from the developer replenishment container to the developing device until a toner amount in the developing device reaches an appropriate amount. At this time, simultaneously, in order to make the toner in the developing device to have a desired charge amount, an operation of rotating the developing roller is also performed during the replenishment of toner.
However, according to the related art, toner continues to be replenished to the developing device successively at a time of setting the developing device to be used for the first time. Therefore, it is difficult to make toner replenished first and toner replenished later to have a uniform charge amount in the developing device, and toner having an excessive charge amount higher than an appropriate value and toner having a small charge amount lower than the appropriate value are mixed. FIG. 3A shows a toner charge amount distribution on the developing roller immediately after the start of the setting sequence and a toner charge amount distribution on the developing roller after the end of the setting sequence. For measuring the toner charge amount distribution, E-SPART produced by Hosokawa Micron Corporation was used.
As shown in FIG. 3A, when toner having a charge amount higher than the appropriate value and toner having a charge amount lower than the appropriate value are mixed, electrostatic aggregation of toner having an excessive charge amount and toner having a small charge amount occurs, which causes insufficient formation of a toner layer on the developing roller. Then, when a toner layer formation defect occurs on the developing roller due to the electrostatic aggregation at a time of initial image formation after setting of the developing device, the amount of toner applied in a layer formation defective portion becomes large. Therefore, a larger amount of toner flies to the photosensitive member as compared to the other portion of the developing roller. Thus, a defective image such as flecks and streaks is generated on the transfer material in a rotation period of the developing roller, as shown in FIG. 3B. The electrostatic aggregation occurs particularly in a low-humidity environment in which the charge amount of toner tends to become high.